Sound localization is a critically important part of sound detection for all vertebrates. Since vertebrates live in many different habitats, various species may use different cues to localize sound sources. Comparative auditory research will help establish a general biological context within which the hearing of all vertebrates, including humans, can be better understood. Sound localization usually refers to the processes of determination of the position of a sound source and is closely related to two fundamental questions: where does the sound come? and how far is the sound source? The proposed project will focus on the first question. This project is directed at understanding the ability and mechanisms of sound localization by a non-mammalian vertebrate, the fish Astronotus ocellatus. Directional hearing in fish has been studied since von Frisch and Dijkgraaf (1935). Previous behavioral, anatomical, and neurophysiological studies have demonstrated the ability of directional hearing in a few fish species and have give some ideas about possible mechanisms underlying sound localization. However, these data are very limited and are mainly obtained from two fish species: cod (Gadus morhua) and goldfish (Carassius auratus). The overall ability of directional hearing has not been systematically studied in any fish species, and what cues fish use to localize sound sources are still unclear. The proposed work will investigate directional hearing in Astronotus ocellatus using a cardiac conditioning method. The focus is on the following specific questions: 1) What is the directional sensitivity of fish in the horizontal, sagittal, and frontal planes? 2) What is the smallest angle that fish are able to distinguish between two spatially separated sound sources? 3) Are fish able to use the cue of phase difference between sound pressure and particle motion to discriminate sound sources from opposing directions?